Expelling students from school is a common practice among schools. If it were my school and my student, I'd be unhappy that your daughter and the other student were fighting. That would make me far more unhappy than the fact that I got hit as I tried to break up the fight.
Rather than a straight out punishment, I'd want to know some things from these two girls - why is it they think it's appropriate to fight?? We can argue in school but nowhere is it appropriate to fight. I'd want them both to figure out what better to do next time around if they ever have another argument. I'd want to hear from both of them that they understand that fighting is not allowed at school and that next time they'll talk things through rather than fall to fighting.
Because people get hurt when they fight and in this case the teacher was hurt.
But that's me - schools more often just expel kids and without talking things over or trying to help the children involved see how they could have done things differently. Schools often don't put a lot of thought into their decisions. I assume by now your daughter is back in school - it's rare for any school to expel a child for more than three days.